This read-only string reports the fully qualified domain name (FQDN) that a dynamic DNS (DDNS) policy entry keeps updated with the device's current address. Admins use it to see which public hostname is tied to a particular DDNS policy, so they know what name to use to reach the device from outside even as its WAN IP changes. For example, an FQDN of "branch1.dyndns.example.com" here tells an admin that name is what should be used to reach this router regardless of its current dynamic IP.